Saturn

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Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Named after the Roman god Saturn, its astronomical symbol (♄) represents the god's sickle. Saturn is a gas giant with an average radius about nine times that of Earth. While only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive than Earth.

Saturn's interior is probably composed of a core of iron, nickel and rock (silicon and oxygen compounds), surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium and an outer gaseous layer. The planet exhibits a pale yellow hue due to ammonia crystals in its upper atmosphere. Electrical current within the metallic hydrogen layer is thought to give rise to Saturn's planetary magnetic field, which is slightly weaker than Earth's and around one-twentieth the strength of Jupiter's. The outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h (1,100 mph), faster than on Jupiter, but not as fast as those on Neptune.

Saturn has a prominent ring system that consists of nine continuous main rings and three discontinuous arcs, composed mostly of ice particles with a smaller amount of rocky debris and dust. Sixty-two known moons orbit the planet; fifty-three are officially named. This does not include the hundreds of " moonlets" within the rings. Titan, Saturn's largest and the Solar System's second largest moon, is larger than the planet Mercury and is the only moon in the Solar System to retain a substantial atmosphere.

Physical characteristics

Montage roughly comparing the sizes of Saturn and Earth.

Saturn is classified as a gas giant planet because the exterior is predominantly composed of gas and it lacks a definite surface, although it may have a solid core. The rotation of the planet causes it to take the shape of an oblate spheroid; that is, it is flattened at the poles and bulges at the equator. Its equatorial and polar radii differ by almost 10%—60,268 km versus 54,364 km, respectively. Jupiter, Uranus and Neptune, the other gas giants in the Solar System, are also oblate, but to a lesser extent. Saturn is the only planet of the Solar System that is less dense than water; about 30% less. Although Saturn's core is considerably denser than water, the average specific density of the planet is 0.69 g/cm3 due to the gaseous atmosphere. Jupiter has 318 times the Earth's mass while Saturn is 95 times the mass of the Earth, yet Jupiter is only about 20% larger than Saturn. Together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System.

Internal structure

Saturn is termed a gas giant, but it is not entirely gaseous. The planet primarily consists of hydrogen, which becomes a non-ideal liquid when the density is above 0.01 g/cm3. This density is reached at a radius containing 99.9% of Saturn's mass. The temperature, pressure and density inside the planet all rise steadily toward the core, which, in the deeper layers of the planet, cause hydrogen to transition into a metal.

Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium with trace amounts of various volatiles. This core is similar in composition to the Earth, but more dense. Examination of the gravitational moment of the planet, in combination with physical models of the interior, allowed French astronomers Didier Saumon and Tristan Guillot to place constraints on the mass of the planet's core. In 2004, they estimated that the core must be 9–22 times the mass of the Earth, which corresponds to a diameter of about 25,000 km. This is surrounded by a thicker liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen that gradually transitions into gas with increasing altitude. The outermost layer spans 1000 km and consists of a gaseous atmosphere.

Saturn has a very hot interior, reaching 11,700 °C at the core, and the planet radiates 2.5 times more energy into space than it receives from the Sun. Most of this extra energy is generated by the Kelvin–Helmholtz mechanism of slow gravitational compression, but this alone may not be sufficient to explain Saturn's heat production. An additional mechanism may be at play whereby Saturn generates some of its heat through the "raining out" of droplets of helium deep in its interior. As the droplets descend through the lower-density hydrogen, the process releases heat by friction and leaves the outer layers of the planet depleted of helium. These descending droplets may have accumulated into a helium shell surrounding the core.

Atmosphere

The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium. The proportion of helium is significantly deficient compared to the abundance of this element in the Sun. The quantity of elements heavier than helium are not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System. The total mass of these heavier elements is estimated to be 19–31 times the mass of the Earth, with a significant fraction located in Saturn's core region.

Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been detected in Saturn's atmosphere. The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of either ammonium hydrosulfide (NH4SH) or water. Ultraviolet radiation from the Sun causes methane photolysis in the upper atmosphere, leading to a series of hydrocarbon chemical reactions with the resulting products being carried downward by eddies and diffusion. This photochemical cycle is modulated by Saturn's annual seasonal cycle.

Cloud layers

A global storm girdles the planet in 2011. The head of the storm (bright area) passes the tail circling around the left limb.

Saturn's atmosphere exhibits a banded pattern similar to Jupiter's, but Saturn's bands are much fainter and are much wider near the equator. The nomenclature used to describe these bands is the same as on Jupiter. Saturn's finer cloud patterns were not observed until the flybys of the Voyager spacecraft during the 1980s. Since then, Earth-based telescopy has improved to the point where regular observations can be made.

The composition of the clouds varies with depth and increasing pressure. In the upper cloud layers, with the temperature in the range 100–160 K and pressures extending between 0.5–2 bar, the clouds consist of ammonia ice. Water ice clouds begin at a level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185–270 K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3–6 bar with temperatures of 290–235 K. Finally, the lower layers, where pressures are between 10–20 bar and temperatures are 270–330 K, contains a region of water droplets with ammonia in aqueous solution.

Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990, the Hubble Space Telescope imaged an enormous white cloud near Saturn's equator that was not present during the Voyager encounters and in 1994, another, smaller storm was observed. The 1990 storm was an example of a Great White Spot, a unique but short-lived phenomenon that occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice. Previous Great White Spots were observed in 1876, 1903, 1933 and 1960, with the 1933 storm being the most famous. If the periodicity is maintained, another storm will occur in about 2020.

The winds on Saturn are the second fastest among the Solar System's planets. Voyager data indicate peak easterly winds of 500 m/s (1800 km/h). In images from the Cassini spacecraft during 2007, Saturn's northern hemisphere displayed a bright blue hue, similar to Uranus. The colour was most likely caused by Rayleigh scattering. Infrared imaging has shown that Saturn's south pole has a warm polar vortex, the only known example of such a phenomenon in the Solar System. Whereas temperatures on Saturn are normally −185 °C, temperatures on the vortex often reach as high as −122 °C, believed to be the warmest spot on Saturn.

North pole hexagonal cloud pattern

North polar hexagonal cloud feature, discovered by Voyager 1 and confirmed in 2006 by Cassini.

Closer view of the north polar vortex at the centre of the hexagon

A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images. Unlike the north pole, HST imaging of the south polar region indicates the presence of a jet stream, but no strong polar vortex nor any hexagonal standing wave. NASA reported in November 2006 that Cassini had observed a "hurricane-like" storm locked to the south pole that had a clearly defined eyewall. This observation is particularly notable because eyewall clouds had not previously been seen on any planet other than Earth. For example, images from the Galileo spacecraft did not show an eyewall in the Great Red Spot of Jupiter.

The straight sides of the northern polar hexagon are each approximately 13,800 km (8,600 mi) long, making them larger than the diameter of the Earth. The entire structure rotates with a period of 10h 39m 24s (the same period as that of the planet's radio emissions) which is assumed to be equal to the period of rotation of Saturn's interior. The hexagonal feature does not shift in longitude like the other clouds in the visible atmosphere.

The pattern's origin is a matter of much speculation. Most astronomers believe it was caused by some standing-wave pattern in the atmosphere; but the hexagon might be a novel aurora. Polygonal shapes have been replicated in the laboratory through differential rotation of fluids.

Magnetosphere

Saturn has an intrinsic magnetic field that has a simple, symmetric shape—a magnetic dipole. Its strength at the equator—0.2 gauss (20 µT)—is approximately one twentieth than that of the field around Jupiter and slightly weaker than Earth's magnetic field. As a result Saturn's magnetosphere is much smaller than Jupiter's. When Voyager 2 entered the magnetosphere, the solar wind pressure was high and the magnetosphere extended only 19 Saturn radii, or 1.1 million km (712,000 mi), although it enlarged within several hours, and remained so for about three days. Most probably, the magnetic field is generated similarly to that of Jupiter—by currents in the liquid metallic-hydrogen layer called a metallic-hydrogen dynamo. This magnetosphere is efficient at deflecting the solar wind particles from the Sun. The moon Titan orbits within the outer part of Saturn's magnetosphere and contributes plasma from the ionized particles in Titan's outer atmosphere. Saturn's magnetosphere, like Earth's, produces aurorae.

Orbit and rotation

The average distance between Saturn and the Sun is over 1.4×109 km (9 AU). It takes Saturn 10,759 Earth days (or about 29 1⁄2 Earth years), to finish one revolution around the Sun.

The average distance between Saturn and the Sun is over 1.4 billion kilometres (9 AU). With an average orbital speed of 9.69 km/s, it takes Saturn 10,759 Earth days (or about 29½ years), to finish one revolution around the Sun. The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth. Because of an eccentricity of 0.056, the distance between Saturn and the Sun varies by approximately 155 million kilometres between perihelion and aphelion, which are the nearest and most distant points of the planet along its orbital path, respectively.

The visible features on Saturn rotate at different rates depending on latitude and multiple rotation periods have been assigned to various regions (as in Jupiter's case): System I has a period of 10 h 14 min 00 s (844.3°/d) and encompasses the Equatorial Zone, which extends from the northern edge of the South Equatorial Belt to the southern edge of the North Equatorial Belt. All other Saturnian latitudes have been assigned a rotation period of 10 h 38 min 25.4 s (810.76°/d), which is System II. System III, based on radio emissions from the planet in the period of the Voyager flybys, has a period of 10 h 39 min 22.4 s (810.8°/d); because it is very close to System II, it has largely superseded it.

A precise value for the rotation period of the interior remains elusive. While approaching Saturn in 2004, Cassini found that the radio rotation period of Saturn had increased appreciably, to approximately 10 h 45 m 45 s (± 36 s). In March 2007, it was found that the variation of radio emissions from the planet did not match Saturn's rotation rate. This variance may be caused by geyser activity on Saturn's moon Enceladus. The water vapor emitted into Saturn's orbit by this activity becomes charged and creates a drag upon Saturn's magnetic field, slowing its rotation slightly relative to the rotation of the planet. The latest estimate of Saturn's rotation based on a compilation of various measurements from the Cassini, Voyager and Pioneer probes was reported in September 2007 is 10 hours, 32 minutes, 35 seconds.

Planetary rings

The rings of Saturn (imaged here by Cassini in 2007) are the most massive and conspicuous in the Solar System.

False-colour UV image of Saturn's outer B and A rings; dirtier ringlets in the Cassini Division and Enke Gap show up red.

Saturn is probably best known for the system of planetary rings that makes it visually unique. The rings extend from 6,630 km to 120,700 km above Saturn's equator, average approximately 20 meters in thickness and are composed of 93% water ice with traces of tholin impurities and 7% amorphous carbon. The particles that make up the rings range in size from specks of dust up to 10 m. While the other gas giants also have ring systems, Saturn's is the largest and most visible. There are two main hypotheses regarding the origin of the rings. One hypothesis is that the rings are remnants of a destroyed moon of Saturn. The second hypothesis is that the rings are left over from the original nebular material from which Saturn formed. Some ice in the central rings comes from the moon Enceladus's ice volcanoes. In the past, astronomers believed the rings formed alongside the planet when it formed billions of years ago. Instead, the age of these planetary rings is probably some hundreds of millions of years.

Beyond the main rings at a distance of 12 million km from the planet is the sparse Phoebe ring, which is tilted at an angle of 27° to the other rings and, like Phoebe, orbits in retrograde fashion. Some of the moons of Saturn, including Pan and Prometheus, act as shepherd moons to confine the rings and prevent them from spreading out. Pan and Atlas cause weak, linear density waves in Saturn's rings that have yielded more reliable calculations of their masses.

Natural satellites

A montage of Saturn and its principal moons ( Dione, Tethys, Mimas, Enceladus, Rhea and Titan; Iapetus not shown). This famous image was created from photographs taken in November 1980 by the Voyager 1 spacecraft.

Saturn has at least 62 moons, 53 of which have formal names. Titan, the largest, comprises more than 90% of the mass in orbit around Saturn, including the rings. Saturn's second largest moon, Rhea, may have a tenuous ring system of its own, along with a tenuous atmosphere. Many of the other moons are very small: 34 are less than 10 km in diameter and another 14 less than 50 km. Traditionally, most of Saturn's moons have been named after Titans of Greek mythology. Titan is the only satellite in the Solar System with a major atmosphere in which a complex organic chemistry occurs. It is the only satellite with hydrocarbon lakes.

Saturn's moon Enceladus has often been regarded as a potential base for microbial life. Evidence of this life includes the satellite's salt-rich particles having an "ocean-like" composition that indicates most of Enceladus's expelled ice comes from the evaporation of liquid salt water.

History of exploration

There have been three main phases in the observation and exploration of Saturn. The first era was ancient observations (such as with the naked eye), before the invention of the modern telescopes. Starting in the 17th century progressively more advanced telescopic observations from earth have been made. The other type is visitation by spacecraft, either by orbiting or flyby. In the 21st century observations continue from the earth (or earth-orbiting observatories) and from the Cassini orbiter at Saturn.

Ancient observations

Saturn has been known since prehistoric times. In ancient times, it was the most distant of the five known planets in the solar system (excluding Earth) and thus a major character in various mythologies. Babylonian astronomers systematically observed and recorded the movements of Saturn. In ancient Roman mythology, the god Saturnus, from which the planet takes its name, was the god of agriculture. The Romans considered Saturnus the equivalent of the Greek god Cronus. The Greeks had made the outermost planet sacred to Cronus, and the Romans followed suit. (In modern Greek, the planet retains its ancient name Cronus (Κρόνος: Kronos).)

Ptolemy, a Greek living in Alexandria, observed an opposition of Saturn, which was the basis for his determination of the elements of its orbit. In Hindu astrology, there are nine astrological objects, known as Navagrahas. Saturn, one of them, is known as " Shani", judges everyone based on the good and bad deeds performed in life. Ancient Chinese and Japanese culture designated the planet Saturn as the earth star (土星). This was based on Five Elements which were traditionally used to classify natural elements.

In ancient Hebrew, Saturn is called 'Shabbathai'. Its angel is Cassiel. Its intelligence or beneficial spirit is Agiel (layga) and its spirit (darker aspect) is Zazel (lzaz). In Ottoman Turkish, Urdu and Malay, its name is 'Zuhal', derived from Arabic زحل.

European observations (17th–19th centuries)

Robert Hooke noted the shadows (a and b) cast by both the globe and the rings on each other in this drawing of Saturn in 1666.

Saturn's rings require at least a 15-mm-diameter telescope to resolve and thus were not known to exist until Galileo first saw them in 1610. He thought of them as two moons on Saturn's sides. It was not until Christian Huygens used greater telescopic magnification that this notion was refuted. Huygens discovered Saturn's moon Titan; Giovanni Domenico Cassini later discovered four other moons: Iapetus, Rhea, Tethys and Dione. In 1675, Cassini discovered the gap now known as the Cassini Division.

No further discoveries of significance were made until 1789 when William Herschel discovered two further moons, Mimas and Enceladus. The irregularly shaped satellite Hyperion, which has a resonance with Titan, was discovered in 1848 by a British team.

In 1899 William Henry Pickering discovered Phoebe, a highly irregular satellite that does not rotate synchronously with Saturn as the larger moons do. Phoebe was the first such satellite found and it takes more than a year to orbit Saturn in a retrograde orbit. During the early 20th century, research on Titan led to the confirmation in 1944 that it had a thick atmosphere—a feature unique among the solar system's moons.

Modern NASA and ESA probes

Pioneer 11 flyby

Pioneer 11 carried out the first flyby of Saturn in September 1979, when it passed within 20,000 km of the planet's cloud tops. Images were taken of the planet and a few of its moons, although their resolution was too low to discern surface detail. The spacecraft also studied Saturn's rings, revealing the thin F-ring and the fact that dark gaps in the rings are bright when viewed at high phase angle (towards the sun), meaning that they contain fine light-scattering material. In addition, Pioneer 11 measured the temperature of Titan.

Voyager flybys

In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, its rings and satellites. Surface features of various moons were seen for the first time. Voyager 1 performed a close flyby of Titan, increasing knowledge of the atmosphere of the moon. It proved that Titan's atmosphere is impenetrable in visible wavelengths; so, no surface details were seen. The flyby changed the spacecraft's trajectory out from the plane of the solar system.

Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings. Unfortunately, during the flyby, the probe's turnable camera platform stuck for a couple of days and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus.

The probes discovered and confirmed several new satellites orbiting near or within the planet's rings, as well as the small Maxwell Gap (a gap within the C Ring) and Keeler gap (a 42 km wide gap in the A Ring).

Cassini–Huygens spacecraft

Saturn during equinox imaged by the Cassini orbiter

Saturn eclipses the Sun, as seen from Cassini.

On July 1, 2004, the Cassini–Huygens space probe performed the SOI (Saturn Orbit Insertion) maneuver and entered into orbit around Saturn. Before the SOI, Cassini had already studied the system extensively. In June 2004, it had conducted a close flyby of Phoebe, sending back high-resolution images and data.

Cassini's flyby of Saturn's largest moon, Titan, has captured radar images of large lakes and their coastlines with numerous islands and mountains. The orbiter completed two Titan flybys before releasing the Huygens probe on December 25, 2004. Huygens descended onto the surface of Titan on January 14, 2005, sending a flood of data during the atmospheric descent and after the landing. Cassini has since conducted multiple flybys of Titan and other icy satellites.

Since early 2005, scientists have been tracking lightning on Saturn. The power of the lightning is approximately 1,000 times that of lightning on Earth.

In 2006, NASA reported that Cassini had found evidence of liquid water reservoirs that erupt in geysers on Saturn's moon Enceladus. Images had shown jets of icy particles being emitted into orbit around Saturn from vents in the moon's south polar region. According to Andrew Ingersoll, California Institute of Technology, "Other moons in the solar system have liquid-water oceans covered by kilometers of icy crust. What's different here is that pockets of liquid water may be no more than tens of meters below the surface." In May 2011, NASA scientists at an Enceladus Focus Group Conference reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".

Cassini photographs have led to other significant discoveries. They have revealed a previously undiscovered planetary ring, outside the brighter main rings of Saturn and inside the G and E rings. The source of this ring is believed to be the crashing of a meteoroid off two of the moons of Saturn. In July 2006, Cassini images provided evidence of hydrocarbon lakes near Titan's north pole, the presence of which were confirmed in January 2007. In March 2007, additional images near Titan's north pole revealed hydrocarbon "seas", the largest of which is almost the size of the Caspian Sea. In October 2006, the probe detected a 8,000 km diameter cyclone-like storm with an eyewall at Saturn's south pole.

From 2004 to November 2, 2009, the probe discovered and confirmed 8 new satellites. Its primary mission ended in 2008 when the spacecraft had completed 74 orbits around the planet. The probe's mission was extended to September 2010 and then extended again to 2017, to study a full period of Saturn's seasons.

Observation

Amateur telescopic view

Saturn is the most distant of the five planets easily visible to the naked eye, the other four being Mercury, Venus, Mars and Jupiter (Uranus and occasionally 4 Vesta are visible to the naked eye in very dark skies). Saturn appears to the naked eye in the night sky as a bright, yellowish point of light whose apparent magnitude is usually between +1 and 0. It takes approximately 29½ years to make a complete circuit of the ecliptic against the background constellations of the zodiac. Most people will require optical aid (large binoculars or a telescope) magnifying at least 20× to clearly resolve Saturn's rings.

While it is a rewarding target for observation for most of the time it is visible in the sky, Saturn and its rings are best seen when the planet is at or near opposition (the configuration of a planet when it is at an elongation of 180° and thus appears opposite the Sun in the sky). During the opposition of December 17, 2002, Saturn appeared at its brightest due to a favorable orientation of its rings relative to the Earth, even though Saturn was closer to the Earth and Sun in late 2003.

In culture

Saturn, from a 1550 edition of Guido Bonatti's Liber astronomiae.

Saturn in astrology () is the ruling planet of Capricorn and, traditionally, Aquarius.

The Saturn family of rockets were developed by a team of mostly German rocket scientists led by Wernher von Braun to launch heavy payloads to Earth orbit and beyond. Originally proposed as a military satellite launcher, they were adopted as the launch vehicles for the Apollo program.

The day Saturday is named after the planet Saturn, which is derived from the Roman god of agriculture, Saturn (linked to the Greek god Cronus).